Can you talk with your T about what happened? Maybe this T was trying to avoid other group members from following your lead and bailing out too. If the one therapist said it wasn't mandatory then ... I would work consider it strongly recommended but not mandatory. If you are not yet ready to attend this outing with the big obnoxious group, then don't. If you and your T think this particular group is good for you, then I would keep going. Just because you took a pass on this outing doesn't mean you might not feel comfortable later to attend another one.